Prostaglandin I.sub.2 (PGI.sub.2) is a physiologically active natural substance having the following structural formula, which is biosynthesized from Prostaglandin H.sub.2 (PGH.sub.2) in the metabolic process in vivo called arachidonate cascade. ##STR3## (see Nature, 263, 663(1976), Prostaglandins, 12, 685(1976), ibid, 12, 915(1976), ibid, 13, 375(1977) and Chemical and Engineering News, Dec. 20, 17(1976)).
PGI.sub.2 has been confirmed to possess not only a very strong inhibitory activity on blood platelet aggregation but a dissociative activity on blood platelet aggregation, an inhibitory activity on blood platelet adhesion, a vasodilating activity, an inhibitory activity on gastric acid secretion etc. Therefore, it has been considered that PGI.sub.2 is useful for the prevention and/or the treatment for thrombosis, arteriosclerosis, ischemic heart diseases, gastric ulcer, hypertension etc. But its application as a pharmaceutical is limited because of its chemical instability and difficulty in separating the activities according to purpose. Accordingly, various PGI.sub.2 derivatives have been synthesized and much research has been carried out for the maintenance and the separation of the activities. However, satisfactory results have been obtained.
Recently, to solve two problems described above, research for PGI.sub.2 receptor agonists which have a new skeleton which may be useful for the treatment of or for the prevention of the above diseases, in view of PGI.sub.2 receptor level, has been carried out.